Plasmid
A circle of DNA that is self-replicating (autonomously replicating) and distinct from the normal genome of bacteria.
A plasmid contains genes that as a rule are not essential to the growth or survival of the cell.
Some plasmids can integrate into the host genome, can be artificially constructed in the laboratory, and serve as cloning vectors (carriers).
'A section of the worm gene is inserted into a plasmid, a structure for carrying genes into bacteria.' (New York Times, Jan 16, 2003